1. Field of the Invention
The present invention relates to the technical field of touch panels and, more particularly, to a method for increasing accuracy of touch coordinate calculation in a capacitive multi-touch system.
2. Description of Related Art
Current consumer electronics are mostly provided with a touchpad for use as one of the input devices. To meet with the requirement of compactness, a touchpad and a panel are typically combined into a touch panel for users to conveniently input data. Upon the sensing principle, touchpads are divided into four types of resistive, capacitive, surface acoustic wave, and optics, in which the capacitive touch panels are the most popular currently.
A typical capacitive touch panel is driven by sensing the grounded capacitance on each conductor line. Thus, a change of the grounded capacitance is used to determine whether an object approaches the capacitive touch panel, which is known as a self capacitance sensing. The self capacitance or the grounded capacitance is not a physical capacitor, but parasitic and stray capacitance on each conductor line. FIG. 1 is a schematic view of a typical self capacitance sensing. As shown in FIG. 1, at the first period of time, the driving and sensing units 110 in a first direction drive the conductor lines in the first direction in order to charge the self capacitance of the conductor lines in the first direction. At the second period, the driving and sensing units 110 sense the voltages on the conductor lines in the first direction. At the third period, the driving and sensing units 120 in a second direction drive the conductor lines in the second direction in order to charge the self capacitance of the conductor lines in the second direction. At the fourth period, the driving and sensing units 120 sense the voltages on the conductor lines in the second direction.
The typical self capacitance sensing of FIG. 1 connects both a driving circuit and a sensing circuit on the same conductor line in order to drive the conductor line and sense a signal change on the same conductor line to thereby decide the magnitude of the self capacitance. In this case, the advantage includes a reduced amount of data, rapidly fetching one frame row data, and lower power consumption. However, the disadvantage includes that an error decision on a touch point that is easily caused by a floating conductor on a touch panel, and a ghost point effect due to multiple touch points concurrently on the touch panel.
With respect to the capacitive touch panel driving method, it senses the magnitude change of mutual capacitance Cm to thereby determine whether the object approaches the touch panel. Likewise, the mutual capacitance Cm is not a physical capacitor but a mutual capacitance between the conductor lines in the first direction and in the second direction. FIG. 2 is a schematic diagram of a typical mutual capacitance sensing. As shown in FIG. 2, the drivers 210 are arranged on the first direction (Y), and the sensors 220 are arranged on the second direction (X). At the upper half of the first period of time T1, the drivers 210 drive the conductor lines 230 in the first direction and use the voltage Vy—1 to charge the mutual capacitance (Cm) 250, and at the lower half, all sensors 220 sense voltages (Vo—1, Vo—2, . . . , Vo_n) on the conductor lines 240 in the second direction to thereby obtain n data. Accordingly, m*n data can be obtained after m driving periods. In a practical system, the drivers 210 and sensors 220 are integrated into the same integrated circuit (IC) to thereby save the cost.
Such a mutual capacitance sensing can easily determine whether a touch is generated from a human body since a signal generated from a floating conductor is different from a grounded conductor, and when multiple points are concurrently touched. Also, the real position of each point can be found since every touch point is indicated by a real coordinate, so as to easily support the multi-touch application.
However, when an object approaches or touches a touch panel, a serious jitter may appear on the voltage signals sensed by the sensors 220 due to the noises generated on a human body, an environment, and/or a panel. In this case, the calculated touch coordinate is unstable, and the entire signal to noise ratio (SNR) of a touch system is relatively reduced. In addition, at a so-called sensing line direction (i.e., Y direction) in a practical touch system, a touch noise can be induced easily, resulting in affecting the accuracy of a touch coordinate calculation.
Therefore, it is desirable to provide an improved method for increasing accuracy of touch coordinate calculation in a capacitive multi-touch system, so as to mitigate and/or obviate the aforementioned problems.